Copper foils and copper alloy foils (hereinafter collectively called "copper foils") are significantly contributing to the progress of the electric, electronic and related industries. Notably, they are indispensable for the fabrication of printed circuits. Copper foil for printed circuits generally is laminated and bonded to a base of thermoplastic resin board, film or the like at high temperature and high pressure, with or without the aid of an adhesive, printed with necessary circuit patterns to form objective circuits, and then is etched to remove unnecessary portions. Finally, desired elements are soldered in place, and in this way various printed circuit boards for electronic devices are fabricated. Qualitative requirements for the copper foil for printed circuit boards differ with the sides, namely, the surface of the side to be bonded to the resin base (matt side) and the surface of the opposite side not to be bonded to the base (shiny side).
Requirements for the matt side chiefly include:
(1) No possibility of oxidative tarnishing during storage; PA0 (2) Adequate resistance to peeling from the base even after high-temperature heating, wet treatment, soldering, chemical treatment or the like; and PA0 (3) Freedom from so-called lamination spots that can result from lamination to the base and etching. PA0 (1) Good appearance and no oxidative tarnishing during storage; PA0 (2) Good solder wettability; PA0 (3) No oxidative tarnishing upon high-temperature heating; and PA0 (4) Good adhesion to resist. PA0 (A) a highly thermal oxidation-resistant copper foil for printed circuits including a thermal oxidation-resistant treated layer formed on the shiny side of the foil and a Cr-base anticorrosive treated layer formed further thereon, characterized in that a copper plating layer is provided between the shiny side of the foil and the thermal oxidation-resistant treated layer, said copper plating layer being preferably composed of fresh copper layer with a thickness of 1 to 20 mg/dm.sup.2 deposited over the entire surface, PA0 (B) a process for producing a copper foil for printed circuits which comprises treating the shiny side of the foil for thermal oxidation resistance and then subjecting it to a Cr-base anticorrosive treatment, characterized in that the shiny side of the copper foil is copper-plated before the thermal oxidation resistance treatment, PA0 (C) a highly thermal oxidation-resistant copper foil for printed circuits including a thermal oxidation-resistant treated layer formed on the shiny side of the foil and a Cr-base anticorrosive treated layer formed further thereon, characterized in that the shiny side of the foil is an etched surface, and PA0 (D) a process for producing a copper foil for printed circuits which comprises treating the shiny side-of the foil for thermal oxidation resistance and then subjecting it to a Cr-base anticorrosive treatment, characterized in that the shiny side of the copper foil is etched before the thermal oxidation resistance treatment, said etched surface preferably being formed using an aqueous solution of ammonium persulfate. PA0 (A) In the case of copper plating, it is only necessary that fresh copper be deposited on the entire surface of the shiny side. There is no limitation, therefore, to the type of bath to be used. However, from the viewpoint of handling, an ordinary copper sulfate-sulfuric acid bath is the most convenient. The composition of the plating solution and the plating conditions are not exacting either. Desirable ranges for the plating solution composition are Cu.sup.2+ : 20-40 g/l and H.sub.2 SO.sub.4 : 50-100 g/l. With regard to the plating conditions, the higher the bath temperature the better, but a temperature in the range of 30.degree.-60.degree. C. gives satisfactory results. Proper current density cannot be unequivocally defined since it is dependent upon the quantity of electricity to be used. A generally effective range is 1-80 A/dm.sup.2, preferably 20.+-.5 A/dm.sup.1. As for the plating time, a period of about 0.1-10 seconds is appropriate. The copper plating thus deposited improves the thermal oxidation resistance of the foil in proportion to the quantity of copper. However, excessive plating can deteriorate the mechanical properties of the copper foil itself or decelerate the treatment rate. A recommended range is 1-20 mg/dm.sup.2. PA0 (B) In the case of etching, the etching method need only produce a copper surface with a uniform chemical activity. The type of bath and the procedure of etching are not critical. Etching methods are roughly divided into two groups, dry (blast and ionizing radiation) and wet (chemical and electrochemical processes). Accuracy, economy, and other considerations make chemical etching the most expedient. While the type of etching bath is not specially restricted, an aqueous solution of ammonium persulfate well known as an etching solution for copper materials is a good choice. The amount of etching is dictated by the concentration of ammonium persulfate, bath temperature, time, agitating condition, and other factors. A desirable amount of etching is not specifically limited but, in terms of the theoretical mean thickness found from the amount of Cu in the etching bath, it ranges from 0.005 to 0.1 .mu.m. If it is less than 0.005 .mu.m, it does not impart complete uniformity of chemical activity. The larger the amount of etching the better the thermal oxidation resistance the foil acquires. An amount in excess of 0.1 .mu.m is undesirable, however, because it can cause changes in the mechanical properties of the foil itself. More desirably, the amount of etching ranges between 0.01 and 0.05 .mu.m. The ammonium persulfate concentration, bath temperature, time, and agitating conditions for that amount of etching cannot be unequivocally specified. Typically, the amount of etching is 0.03 .mu.m under the following set of conditions:
Requirements for the shiny side include:
As for the thermal oxidation resistance of the shiny side, the requirement has become more and more stringent in recent years. For one thing, copper foils have come to be exposed to higher temperatures than heretofore because of the advent of novel, highly heat-resistant resins as well as a new manufacturing method which is known as the "double-layer flexible base process". This method comprises directly applying a polyimide varnish to a copper foil, thereby forming a double-layer structure of polyimide and copper foil layers. With conventional lamination techniques too, there is a tendency toward the replacement of the usual nitrogen atmosphere by air to reduce the cost of laminating and curing processes that involve heat treatment. This is another factor demanding an improvement in the thermal oxidation resistance of the shiny side of copper foil. The shiny side to be printed with minute electric circuits ought to be protected against tarnishing or other deleterious effects of oxidation. To be more concrete, the shiny surface today is required to undergo no tarnishing on holding at 300.degree. C. for 30 minutes.
For the surface treatment of the shiny side to meet all the foregoing requirements, a method combining zinc plating with chromate treatment has predominantly been used. However, the thermal oxidation resistance that the method imparts is limited, e.g., just enough for holding at 200.degree. C. for about 30 minutes. The resistance increases in proportion to the amount of zinc plating used, but it is known to present problems of yellowing ("brassing") after bonding and of low printability due to decreased resistance to acid.
Another method reportedly under development replaces the zinc plating with zinc-nickel plating, zinc-cobalt plating or the like for the formation of a zinc alloy layer as a thermal oxidation-resistant treated layer, followed by chromate treatment. Even the proposed method is unable to make the shiny side resistant to thermal oxidation under the conditions of 300.degree. C. and 30 minutes; it imparts a resistance to tarnishing for at most 30 minutes at 240.degree. C. or 10 minutes at 270.degree. C.